The present invention relates to permanent, full color transparencies, each having more than one imaged dichroic filter and an imaged neutral density filter deposited on a single glass substrate, and methods for making the same.
Conventional color transparencies generally fade over time due to the degradation of their photosensitive organic dyes. The degradation of these transparencies is particularly severe when they are used in connection with high intensity projectors which emit visible and ultraviolet radiation that is absorbed by the dyes. Thus, the color balance and intensity of a transparency will eventually change if it is continuously projected for long periods at a time, such as those transparencies which are utilized in amusement park exhibits and the like. However, it is not uncommon for transparencies containing organic dyes to obtain uneven color fading within a short period of time, such as after eight hours of continuous projection under high illumination. Accordingly, these transparencies must be replaced as often as once a week, thereby requiring additional costs and maintenance to operate.
In order to avoid color fading, it is generally known in the prior art that a dichroic filter, made from layers of dielectrics such as metal oxides, can be utilized to generate a single color transparency on a glass substrate instead of using organic dyes. Unlike organic dyes, the dielectrics of the dichroic filter are capable of withstanding the effects of high illumination systems. Color degradation and fading is avoided since the dielectrics reflect rather than absorb visible light and ultraviolet radiation. However, the full color transparencies known in the prior art which use dichroic filters have done so using multiple glass substrates. In other words, the color transparencies having more than one color must be made from multiple glass substrates each having a dichroic filter deposited on its surface. The color images on these dichroic filters are of low resolution and are responsible for the hue of the colors. Thus, an additional fourth glass substrate having a black neutral density filter, created by a single layer of metal, is necessarily used to regulate the brightness of the image and to carry higher resolution details of the image. The multiple layers of dichroic filters plus the neutral density filter, each on separate glass substrates, are adhesively bonded together to produce the overall full color transparency. Examples of such transparencies are disclosed in an article entitled "Nondegrading Color Transparencies," by van Milligen et al., in Applied Optics, Vol. 24, No. 4, Feb. 15, 1985.
It has been found, however, that color transparencies having dichroic filters on multiple substrates are not acceptable because of their many disadvantages. First, the use of multiple substrates produces multiple focal planes which cause interference and misalignment of colors. Therefore, the image cannot be properly focused because of the inherent lack of a single focal plane. Also, in this regard, it is difficult to align images by adhesively bonding the multiple substrates on which the images appear within the required tolerances for high resolution. Further, adhesively bonding the substrates also introduces the potential for delamination under heating and cooling cycles associated with high intensity projectors.
In addition, these color transparencies do not provide high resolution color images in each of the three subtractive primary colors, magenta, cyan, and yellow. High resolution color images in each of these primary colors is essential for an overall high resolution color transparency. This cannot be achieved solely through the use of a high resolution neutral density filter as simply described above, and as commonly used in the prior art.
Further, the processes known in the prior art for preparing permanent color transparencies commonly use an etching sequence which is not controlled to protect the substrates or filters. Among other things, this may result in non-uniform coloring which is particularly apparent on the magenta layer.
Accordingly, there has existed a definite need for permanent, full color transparencies which overcome these problems. More particularly, there is a need for a permanent, full color transparency on a single glass substrate which provides a high resolution color image, having a single focal plane, which will not fade when projected for long periods of time by high intensity projectors.